1. Field of the Invention
The present invention relates to test jigs and related test equipment apparatus for extremely high speed transmission of signals.
2. Description of the Technology
Recent developments in the design and fabrication of integrated circuits have resulted in devices which produce signals which propagate faster than most test equipment is capable of detecting or measuring. Previous generations of high speed digital circuits could be evaluated for switching rise times and other output pulse characteristics by measuring equipment which was able to resolve pulses occurring over time periods lasting only billionths of a second. These intervals, now commonly referred to as nanoseconds, have become relatively long passages of time compared to the intervals spanned by signals generated by state-of-the-art digital circuitry. Transmission lines which convey these signals must now cope with waveforms having periods below the nanosecond range. The transmission of this electrical energy from the signal source to a receiving device poses new and difficult problems which were not encountered with apparatus operating below the one billion cycles per second or gigahertz frequency range.
Distortion caused by transmission pathways is proportional to the switching speed of the signals and to the length of the pathway. A significant portion of this distortion can be avoided by employing flat cable connectors, twisted pairs of conventional wire, or coaxial conductors. The distortion encountered when these prior pulse handling devices are used to convey gigahertz waveforms simply overwhelms the signal. A major component of this distortion is due to improper impedance matching. When impedance discontinuities are present in a test circuit configuration, unwanted reflections are created within the transmission pathway which degrade the signal by effectively cancelling out the transfer of energy from the signal source to the receiving circuit. These reflections arise when a signal encounters a sudden change in impedance somewhere along a transmission line. A simple technique which reduces these reflections is shortening the length of the interconnections or spacing the conducting lines more closely, but these two remedies create a concomitant disadvantage, in that they greatly enhance the problem of crosstalk, which is discussed below in detail. In a circuit in which the impedance is perfectly matched, both the source and the load at opposite ends of the transmission pathway match the characteristic impedance of the pathway.
Another type of distortion which corrupts signals is crosstalk. Fields of radiation which are propagated by electrical current flowing through transmission lines can induce currents in other nearby conductors. Both electrostatic and electromagnetic fields can produce unwanted interference in signal conveying transmission lines. A primary coupling mechanism which creates crosstalk is the capacitance between closely spaced conductors. Conductors which are near to each other develop a high mutual capacitance. Higher signal frequencies cause an even higher amount of energy to be transferred via capacitive coupling. Conventional shielding, which completely encloses a transmission line within a conductor which is grounded, can reduce a large share of this type of distortion caused by radiation, but extremely high signal frequencies tend to defeat such protective measures. One way to minimize crosstalk is by separating transmission lines by relatively large distances, but design constraints and countervailing sources of distortion which would be proportionally increased by larger line separations militate against using this tactic to reduce this source of noise. Another simple means of reducing crosstalk is to form a helical arrangement of pathways using two common insulated wires which are tightly twisted together. Althouth the added path length brought about by the winding of the conductors adds time delays and other forms of distortion, radiation from each wire in such a twisted pair substantially cancels that emitted by the other and thus cuts down on crosstalk.
Flat cables comprising insulated, round or flat wires which are bound together to form integrated, ribbon-like strips are also employed in attempts to mitigate crosstalk distortion. Conductors which lie side by side, and which are kept apart and are sealed within an insulating material, can be segregated with alternating ground wires between the signal carrying lines. The diminution in crosstalk achieved by this technique is directly proportional to the number of ground wires interspersed among the transmission wires.
Techniques which address the problems of impedance mismatches and crosstalk are descirbed by James Fetterolf in an article entitled "Transmission-line Methods Speed 1-ns Data Along" published in the June 21, 1980 edition of Electronic Design on pages 95 through 99. This publication is geneally concerned with prior art technology which can be employed to construct single layer printed circuit boards in which unreliable jig delays and uncertain propagation times cause severe difficulties in accurately evaluation test equipment.
In U.S. Pat. No. Re. 41,477, Marshall describs a multi-signal transmission line formed of a flat cable having a plurality of generally parallel conductors embedded in a dielectric core material. Marshall asserts that his composite transmission line cable reduces far end line-to-line interference between the signal conductor and adjacent quiet lines.
U.S. Pat. No. 4,283,694--Pauker discloses an impedance matching microstrip network for matching a predetermined impedance to a lower impedance over a wide frequency band. This impedance matching device is used in a Gunn-diode oscillator which is tuned over a wide frequency band by means of a small ball of yttrium-iron garnate which is placed in a static magnetic field.
Gehle discloses a microwave integrated circuit in an assembly having a structurally continuous ground plane of conductive material in U.S. Pat. No. 4,184,133. The microwave integrated circuit substrate is mounted to a carrier or chassis by a layer of dielectric material interposed between the lower surface of the substrate and the carrier.
McAvoy describes a resonant ring transmission line having a high Q mode in U.S. Pat. No. 3,967,223. This invention comprises a resonant ring transmission line coupled to a microstrip transmission line.
None of the preceding inventions solves the problems of the deleterious effects of crosstalk and imepedance matching to an extent that would allow for the reliable testing of integrated circuit devices which proprogate gigahertz frequency signals. None of the prior art devices provides an effective and relatively inexpensive solution to the electrical distortions described above in detail which plague the operation of currently available test equipment. Such a solution would satisfy a long felt need manifested by the current efforts of the electronics test equipment industry, which continues to attempt to develop measurement systems which can cope with the ever-increasing speed of operation of new integrated circuit devices. The development and manufacture of extremely high speed integrated circuit systems has generated a concomitant need for an invention which is capable of measuring signals from these devices.
Such an invention would ideally be suited to function effectively with many different integrated circuit packaging configurations and would be easily adapted to a variety of automatic test equipment systems. Such an innovative, new test jig would be capable of subjecting a test device to extreme ranges of temperature without causing the degradation of test data. This new test jig would expand the bandwidth of currently available test equipment to the gigahertz range while eliminating the tedious and extremely inconvenient standard practice of subtracting unreliable jig delays from final measured test data values.